Improve tree-shaking:

[mraleph]

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• Use dart2js:prefer-inline everywhere where we use vm:prefer-inline.
• Make platformOS const/platform-const by wrapping version into a thunk. This means you don't have to lazy initialize the final field.
• Allow full constant folding of OperatingSystem.current if overrideOperatingSystem is never called in the program by going through an indirection. Without this indirection a bunch of goo stays behind after each invocation of OperatingSystem.current in the program